If supported, multi room reservations can be uploaded in a single reservation XML. Each room is designated its own RoomStay element.
Example of a reservation with 2 of the same room type booked, with a separate guest in each room.
<OTA_HotelResNotifRQ xmlns="http://www.opentravel.org/OTA/2003/05" EchoToken="4528545b-f875-5c77-9205-22631b68f15a" TimeStamp="2018-07-31T01:16:16Z" Version="1.0">
<HotelReservations>
<HotelReservation CreateDateTime="2018-07-31T01:16:16Z" LastModifyDateTime="2018-07-31T01:16:16Z" ResStatus="Reserved">
<POS>
<Source>
<RequestorID ID="pmsprovider"/>
</Source>
<Source>
<BookingChannel Type="1" Primary="true">
<CompanyName>Booking.com</CompanyName>
</BookingChannel>
</Source>
</POS>
<UniqueID ID="529269fb1459"/>
<RoomStays>
<RoomStay>
<RoomTypes>
<RoomType RoomID="3" RoomType="King Room" RoomTypeCode="20145">
<RoomDescription>
<Text>King Room</Text>
</RoomDescription>
</RoomType>
</RoomTypes>
<RatePlans>
<RatePlan RatePlanName="King Room Standard" RatePlanCode="42951" EffectiveDate="2018-08-06" ExpireDate="2018-08-09">
<RatePlanDescription>
<Text>King Room Standard</Text>
</RatePlanDescription>
</RatePlan>
</RatePlans>
<RoomRates>
<RoomRate EffectiveDate="2018-08-06" ExpireDate="2018-08-09" RoomTypeCode="20145" RatePlanCode="42951" RoomID="3" NumberOfUnits="1">
<Rates>
<Rate UnitMultiplier="1" EffectiveDate="2018-08-06" ExpireDate="2018-08-07">
<Base AmountAfterTax="349" CurrencyCode="USD"/>
<Total AmountAfterTax="349" CurrencyCode="USD"/>
</Rate>
<Rate UnitMultiplier="1" EffectiveDate="2018-08-07" ExpireDate="2018-08-08">
<Base AmountAfterTax="349" CurrencyCode="USD"/>
<Total AmountAfterTax="349" CurrencyCode="USD"/>
</Rate>
<Rate UnitMultiplier="1" EffectiveDate="2018-08-08" ExpireDate="2018-08-09">
<Base AmountAfterTax="299" CurrencyCode="USD"/>
<Total AmountAfterTax="299" CurrencyCode="USD"/>
</Rate>
</Rates>
</RoomRate>
</RoomRates>
<GuestCounts>
<GuestCount AgeQualifyingCode="10" Count="2"/>
</GuestCounts>
<TimeSpan Start="2018-08-06" End="2018-08-09"/>
<Total CurrencyCode="USD" AmountAfterTax="997"/>
<ResGuestRPHs>
<ResGuestRPH RPH="1"/>
</ResGuestRPHs>
</RoomStay>
<RoomStay>
<RoomTypes>
<RoomType RoomID="4" RoomType="King Room" RoomTypeCode="20145">
<RoomDescription>
<Text>King Room</Text>
</RoomDescription>
</RoomType>
</RoomTypes>
<RatePlans>
<RatePlan RatePlanName="King Room Standard" RatePlanCode="42951" EffectiveDate="2018-08-06" ExpireDate="2018-08-09">
<RatePlanDescription>
<Text>King Room Standard</Text>
</RatePlanDescription>
</RatePlan>
</RatePlans>
<RoomRates>
<RoomRate EffectiveDate="2018-08-06" ExpireDate="2018-08-09" RoomTypeCode="20145" RatePlanCode="42951" RoomID="4" NumberOfUnits="1">
<Rates>
<Rate UnitMultiplier="1" EffectiveDate="2018-08-06" ExpireDate="2018-08-07">
<Base AmountAfterTax="349" CurrencyCode="USD"/>
<Total AmountAfterTax="349" CurrencyCode="USD"/>
</Rate>
<Rate UnitMultiplier="1" EffectiveDate="2018-08-07" ExpireDate="2018-08-08">
<Base AmountAfterTax="349" CurrencyCode="USD"/>
<Total AmountAfterTax="349" CurrencyCode="USD"/>
</Rate>
<Rate UnitMultiplier="1" EffectiveDate="2018-08-08" ExpireDate="2018-08-09">
<Base AmountAfterTax="299" CurrencyCode="USD"/>
<Total AmountAfterTax="299" CurrencyCode="USD"/>
</Rate>
</Rates>
</RoomRate>
</RoomRates>
<GuestCounts>
<GuestCount AgeQualifyingCode="10" Count="2"/>
</GuestCounts>
<TimeSpan Start="2018-08-06" End="2018-08-09"/>
<Total CurrencyCode="USD" AmountAfterTax="997"/>
<ResGuestRPHs>
<ResGuestRPH RPH="2"/>
</ResGuestRPHs>
</RoomStay>
</RoomStays>
<ResGuests>
<ResGuest ResGuestRPH="1" ArrivalTime="17:00:00" PrimaryIndicator="true">
<Profiles>
<ProfileInfo>
<Profile ProfileType="1">
<Customer VIP_Indicator="false">
<PersonName>
<GivenName>John</GivenName>
<Surname>Smith</Surname>
</PersonName>
<Telephone PhoneNumber="0312456789" PhoneTechType="1"/>
<Email EmailType="1">email@test.com</Email>
<Address Type="1">
<AddressLine>123 Test Street.Unit 1</AddressLine>
<CityName>Testville</CityName>
<PostalCode>3456</PostalCode>
<StateProv>VIC</StateProv>
<CountryName>Australia</CountryName>
</Address>
</Customer>
</Profile>
</ProfileInfo>
</Profiles>
</ResGuest>
<ResGuest ResGuestRPH="2">
<Profiles>
<ProfileInfo>
<Profile ProfileType="1">
<Customer VIP_Indicator="false">
<PersonName>
<GivenName>Jane</GivenName>
<Surname>Smith</Surname>
</PersonName>
</Customer>
</Profile>
</ProfileInfo>
</Profiles>
</ResGuest>
</ResGuests>
<ResGlobalInfo>
<TimeSpan End="2018-08-09" Start="2018-08-06"/>
<Comments>
<Comment GuestViewable="true">
<Text>Seaside view if possible, as it is my birthday weekend</Text>
</Comment>
</Comments>
<Total AmountAfterTax="1994" CurrencyCode="USD">
<Taxes CurrencyCode="USD" Amount="181.27">
<Tax Amount="181.27" Percent="10" CurrencyCode="USD">
<TaxDescription>
<Text>GST</Text>
</TaxDescription>
</Tax>
</Taxes>
</Total>
<HotelReservationIDs>
<HotelReservationID ResID_Type="10" ResID_Source="PMS" ResID_Value="529269fb1459"/>
<HotelReservationID ResID_Type="14" ResID_Source="BDC" ResID_Value="RES123456"/>
</HotelReservationIDs>
<Profiles>
<ProfileInfo>
<Profile ProfileType="1">
<Customer VIP_Indicator="false">
<PersonName>
<GivenName>John</GivenName>
<Surname>Smith</Surname>
</PersonName>
<Telephone PhoneNumber="0312456789" PhoneTechType="1"/>
<Email EmailType="1">email@test.com</Email>
<Address Type="1">
<AddressLine>123 Test Street.Unit 1</AddressLine>
<CityName>Testville</CityName>
<PostalCode>3456</PostalCode>
<StateProv>VIC</StateProv>
<CountryName>Australia</CountryName>
</Address>
</Customer>
</Profile>
</ProfileInfo>
</Profiles>
<BasicPropertyInfo HotelCode="7294"/>
</ResGlobalInfo>
</HotelReservation>
</HotelReservations>
</OTA_HotelResNotifRQ>