If you have dual-tuner receiver, this receiver can be used permanently any time you like. It does not mean that with dual-login, you will use receiver 1 today, receiver 2 tomorrow (or other time). Both tuners within one receiver are used simultaneously. as a rule, tuner 1 - for watching TV, tuner 2 - for recording some other TV channel.
If you mean 2 stand alone receivers - they cannot be used simultaneously. Only 1 receiver with dual-tuner.