i thought so too . but maybe just maybe he was in love with akane first but then mai confessed and he couldn't say no . but then mai died and he couldn't leave akane in such a tragic situation coz he loves him . and akane thinks the same. but the lack of communication extend the situation more . thats what i got from it . and it feels like they were acting according their emotions not logic . sorry for talking too much
There are a few possibilities, but the one I fear is that in the grief to try to keep Mei alive he projected his feelings onto someone of his likeness which is Akane. This is possible since Even I have had this happen to me and got to say it's not fun to deal with, but with Akane in the state where he wants to become Mei and his where he projects Mei would be both toxic and work together. Though if this was the case then he wouldn't call Akane his own name. So I am hoping it is more Monohara's idea, which is another possibility and one I favor more.

I'm a bit disgusted by the seme, who dates their deceased boyfriend's identical twin brother, that's just disturbing especially since it's implied they got together right after the bro died.