Now what about a 4 ohm head and two 4 ohm cabs?
No. When you plug two 4 ohm cabs into the head, the head will see 2 ohms. The speaker outputs on an amp are wire in parallel, so two 4 ohm cabinets will be 2 ohms, two 8 ohm cabinets witll be 4 ohms, etc...
All I have is a head and two 8x10s (which I'm assuming are 4 ohms.) I don't know how many ohms the heads at my disposal are.
If the head is a tube head, you can't assume anything. You have to match the impedance with the head. If it is a transistor head, then you can have anything above the minimum impedance.
I have two 4 ohm 8x10s. What's the safest way to connect them to a head (to simplify, how many ohms does my head need to be to safely run two 8x10s in PARALLEL? SERIES? TWO OUTPUTS, TWO CABS. One output, two cabs.
IF your cabinets are 4 ohms and you head can handle 2 ohms, just plug the two cabinets in and you will be fine.
If the amp can only handle 4 ohms, you can only use one of the cabinets (assuming you are right and they are 4 ohms each).
It doesn't matter if you run two outputs on the head to two cabinets or you just run one output to the first cab and daisy chain to the second one, it is still 2 ohms because they will still be wired in parallel either way.
If you are talking about Ampeg 8x10 cabs, then they are 4 ohms. If you are powering them with a newer (not from the 70's) SVT, it should handle a 2 ohm load. But don't take my word for it, they did different things in different era's. They made 8 ohm 8x10 cabs so that you could run four of them off of one head. Way back, they made 8 ohm cabs so that you could run two off of the SVT from the 70's that (If I remember correctly) only went down to 4 ohms.
If you built your own speaker cable to wire both cabinets in series, that would get you an 8 ohm load. However, if you screw it up, you will blow the amp.