I'm assuming the best way to control tap tempo for a Timefactor and Modfactor chained together is to use an external switch (such as the Digitech FS300) with a Y cable going to each device. For example, I'd like to set the phaser in Modfactor and digital delay in Timefactor to the same tempo when I tap on the switch. Am I correct in the best way to achieve this?
Yes, this works and is the best way to do what you are asking.
Using a Y cable on the Aux Switch you could even set one of the three switches to tap tempo on TimeFactor, the next switch to tap tempo on ModFactor, and the third switch to tap tempo on both. Here's how you would do that:
TimeFactor AUX settings:
TAP > TIP FS3 > T+R
ModFactor AUX settings:
TAP > RNG FS3 > T+R
Just keep both stompboxes in Bank Mode for this to work correctly.