Hi, Eric. I concur with brother Stephen. The problem with using a preamp, like the F-2B, in front of a combo like your Fenders, is that the signal from the preamp will be much too hot for the Fender to handle. You use a preamp paired to a power amp connected to speaker(s). Or you put them all in one box and you have a "combo" amp, like your Fenders.
I'm a guitar player. My main rig is an F-2B into a Carvin DCM300L (4 lb. class D stereo power amp - 100w/ channel into 8 ohms - one rack unit), into a pair of 1x12 cabs with JBL K120, one with a hemp cone. I plug my guitar into into input "1" on The "A" channel of the F-2B. Then I "jumper" a cable from "A's" input "2" the channel "B's" input "1". I then mix one channel to accentuate the highs and the other, with the darker sounding hemp cone, to accentuate the bass. I love the F-2B as I'm a Fender guy and the F-2B is based on the old Fender Dual Showman.
What are you trying to achieve? If it's to quickly and efficiently change from guitar to bass, and vice versa, there are foot switchers that will allow you to easily switch between guitar and bass, including between your guitar and bass amps.
Bill, the guitar one