I too miss this option. Now, I switch between bunq and calculator app which is annoying. Some ideas to improve this.
1) Apart from 'request' and 'split', maybe there could be an option to 'select & sum' multiple transaction and only then request or split.
2) A more feature rich transaction calculator which can handle calculations such as A + B/2 - C + 50 (where A, B & C are different transactions). And only then 'request' or 'split'.
I know Splice is a way to do this. However, most of the time I find that I just need something simpler that only involves my transactions. Furthermore, you can use both options to gain more insight in your spending. I often sum some transactions for this reason.