no, bad idea, you will either have to undervolt and underclock the ram you have, or overvolt the new ram, which could destroy it. What is the ram your currently running? It may be able to be undervolted, but would need to be underclocked to be stable. There's plenty of 1.65v ram on the market. Safest bet is to remove your old ram and only use the new ram. 8gb is plenty, the extra 4gb wont see you any difference. If anything the older ram with different timings/voltages will just slow the new ram down and cause instability.
Slowing things down will not cause instability. This is not about halving the frequencies. This is about just 10-20% less. And actually 1.65V is overclocked memory.
Oh, and since the OP asks these questions, I doubt he actually uses 1.65V, since it's also dangerous to the CPU so the BIOS wil not set it by default.