Can the two outputs of MAX20034 be connected together and used as multi-phase single-output?

The IC isn't inherently designed for multi-phase operation, but because the channels are conventional current-mode-regulated supplies, you could probably parallel the phases. You would need to tie the FB pins together, tie the COMP pins together, and adjust compensation accordingly.

Thanks for your confirmation. 

Is there any potential risk in this usage?

The biggest risk is sub-optimal current-sharing between channels. Tying the channels' control lines together (as mentioned previously) should help avoid drastic imbalances, but it will likely not be as well-matched as regulators designed for multiphase operation.

The best approach would be to acquire an EV Kit and test it out on the bench.