You can do whatever you want!
The stuff is yours right? of course it is!!
Yes it's safe - the reason that it's recommended NOT to connect subs with different ohm loads and such to the same amp is because for your system to sound optimal, everything should match.
What you have to do though is go by the sub with the lowest power handling. If you go off of the other sub, then you'll blow the weaker sub, but if course if the more powerful sub is underpowered, then it isn't going to sound as good as it's supposed to.