but really any mtb will help you improve your mtb skills. Hardtail or full suspension - riding technical terrain on any bike translates to riding technical terrain on the cross bike. Now that I typed that, riding technical terrain on your cx bike also helps with your cx skills. So a mtb is not necessary but it sure is fun.
I think even a hardtail 29er is a great option. And, I second what Frank stated: ride your CX bike on MTB trails to improve your skills. Likely not going to ride gnarly rocky stuff, but smooth single track with a few technical sections here and there is perfect. If the a section is too technical, just get off an run…like you would in a CX race!
I joined the FoCo Fondo Fest Gravel pre-ride last weekend led by one Zach Allison of Team Cliff Bar. Zach took us up to Red Mountain Open Space near the Colorado boarder where we were treated to 9 miles of beautiful single track. Some of it was a bit rocky and I had to really choose lines extra carefully riding a CX bike with 35c tires. Any MTBing skills I had really came into play and I’m sure the experience only improved them! That section was so much fun (even though I was deep in the back of the sufferbus having not paced slowly enough in the first 25 miles), but I was happy to be back on plain old dirt roads after Red Mtn.