5V Across Multiple USB Ports
I think it's fine, and I'd just connect them.

However, if you want to be extra safe, note that some USB voltages can be quite a bit lower than others, due primarily to the drops in the current limiting devices in each hub.  Or you could invent even stranger cases, like having one plugged into the end of a chain of bus-powered hubs, and the other plugged directly into a self-powered hub.  Shorting the +5V signals together may end up causing current to flow backwards in some of the hubs, if one device's voltage is higher than the other.  Active electronics, like hubs that support power switching, may be damaged by that.

Instead, you can connect each +5V through a diode to your +V rail on your circuit.  Your circuit's +V would be dropped a bit (maybe 0.6V for a 1N4148 or similar diode), but the USBTiny and FT232R should likely both be OK with that.