I guess it's fine to show return leg tickets BG - BP and the second ticket of BP - Bratislava?
No, Russian Railways (RZD) became big fans of global price tickets, so ordinary SCIC-NRT tickets won't work. As far as I know, special tariff for Bratislava-Belgrade is 43,3 euro (2nd class) or 64,8 euro (1st class) + sleeper reservation (see below)
Do you know how much the sleeper is for 2 people (sharing a 4 berth is OK if it's a lot cheaper)
2 bed sleeper (double) costs 26 euro and requires 1st class ticket, 2/3 or 2/4 (tourist) costs 17 euro and requires 2nd class ticket.
--> it meas that in case you want to travel using RZD carriages, then it will cost you minimum 60,3 euro (43,3+17)