It depends a bit on the make. Spektrum have a range of Receivers for the DX range of Transmitters. On non-memory Txs, you can have as many Rxs of the same model as you want assigned to one Tx. With memory TXs you can have a different Rx model for each stored boat model.
I'm not sure of the situation with Hitec & Futaba. Here in the UK the cheap Chinese sets which are sold under various brand names only seem to have one model of receiver for them, so they can have as many Rxs as you want assigned to one TX.
Can you sail two models at the same time with one Tx - well that depends on your powers of concentration
Barrie