Please forgive what may be a really basic question, but is it safe to use both 5v and 3.3v at the same time with a shared/common Gnd? If not, what is recommended method?
Absolutely no problem. Passing 5V power or signals to a 3V chip on the otherhand
could cause damage, so make sure everything gets the right supply and appropriate
level shifting circuitry is used.
Ground is just a reference point.
In general, there are 5V-tolerant 3.3V logics. You have to look into the data sheets. If you use 3.3V to drive a 5V input you shouldn't have problems (even when you consider the different definitions of LOW and HIGH levels).
Levelshifting could be done by some logic circuits (e.g. series LVx from Texas Instruments). A good comparison you find here Logic & voltage translation | TI.com
Nevertheless, you also could use Diodes and Transistors like below. The R provides the HIGH level of 3.3V while the 5V (or 9V or or or) provides the LOW. The Value of R could be also 100k or 1k. This depends on the speed you need - but this is another issue.
CPARKTX:
Please forgive what may be a really basic question, but is it safe to use both 5v and 3.3v at the same time with a shared/common Gnd? If not, what is recommended method?
It's unsafe to NOT share the ground...
CPARKTX:
but is it safe to use both 5v and 3.3v at the same time with a shared/common Gnd?
I suppose it depends on what you mean by "use"
If you don't share grounds then you have to do some fancy electronics instead like optical coupling or transformers.