I see what you mean, yes I tried and looks like the animations can’t be blended but played one after another, and if you add another Slot to Montage and a new animation, I don’t see any settings for blending per bone or anything like that, the first animation simply overrides everything else.
Well maybe if you would try to blend per bone like in the video:
Jump to about 06:00 and watch from there, he created BlendPerBone slot and names the bone to blend, so he blends both animations, I didn’t try it but it looks like this should work.Also to time it all pefectly you could make 2 montages out of your 2 recorded animations, you can add a Montage Section to the track and name it, so that can help specify where your animations start playing (using PlayAnimMontage you can specifiy “Start Section Name” ). You could even use AnimNotify on the base animation to start the 2nd animation at right time.
I hope you can make it work, good luck :P.