You can have as many receiver you want to pay for. I have 3 receivers on my main TV (HR20, a DirecTivo and an old Hughes E-86). DirecTV will happily take your $5 a month extra receiver fee.
If you deactivate a leased receiver you have to return it to Directv. If the D11 is working correctly there is little reason to replace it with a D12. The main difference between the two is the ability of the D12 to use the upcoming SWM (single wire multiswitch) but with a single receiver there is no reason to do that.