Honestly it didn't look like they were "holding back" that much, instead it seems they were being overconfident thinking it would be easy, then progressively started to have a harder and harder time until Shen decided to end things because they were having trouble, to then get genuinely worried when his trick failed since he had no clue about what happened at first.
They only contained Mc thanks to Shen, everything else wasn't working, yes the plan was to capture but by the end Devana had enough and was ready to kill, this shows how things went way out of proportion for them.
I don't think they would be able to even secure a kill whitout Shen, i don't know if Mc was ready to win a fight against both but i believe he definitly could survive the encounter.
They were definitely holding back and not just because mom said it, Devana herself admitted that she was just fucking around and was about to summon a monster much more dangerous than before if not for Shen stopping her.
And I don't quite understand what do you mean by harder time, as through out the whole fight the only successful damage MC inflicted on the twin is only due to element of surprise, as the twin though Shen had subdued MC and were ill prepared for his sudden retaliation, it's noting more than a lucky shot really.
If they were to go all out from the start, Devana will summon her doggy that was dangerous even for MC for a 3v1, Evander will start using name moves like when he was against Met, and if the worst come to it one of them can combine both of their power to utilize moves far above that of a normal lv 3 caliber. Fighting the twin alone is an extremely dangerous endeavor even for MC and definitely not as easy as some of the people here might think.
In this updated even the MC admit that the smarter approach of fighting them will be having another equally powerful ally to fight them simultaneously.