Short answer is yes, you'll need the FS3x however, even though you can save loop 1 while it is playing, you would have to:
1) stop the looper, advance to the next slot, then play and record, save that loop 2 to an ajacent memory location. Then you can switch between the two loops.
2) get a 2nd looper and two FS3x's. Put the FS3x for Looper A next to looper B in such a way as to allow one stomp to stop Looper A while also starting Looper B. Same with the FS3x for Looper B, it goes next to Looper A.
You can see this in some of the photos on my Facebook pages, if we're connected.
This is a short answer but I hope it covers your question. I know what you are trying to accomplish - it is similar to my needs. Now that I have (5) XT's sync'd and my old non-XT also on the floor, I can accomplish what you're seeking.
There is also a DIY mod that I came up with to allow 'saving' with your foot instead of bending down.
> From: firstname.lastname@example.org
> To: Loopers-Delight@loopers-delight.com
> Subject: DigiTech JamMan Express XT /
> Date: Tue, 11 Nov 2014 01:40:25 -0600
> Hi all,
> Quick question, does the Jamman Express XT allow you to record say one
> progression, store it, then make another progression, and then switch
> between the two handsfree? Or do I need the extra pedal they sell?