Hey there, so your question is not so much about feeding as about how to deal with DD and feeding? Is that right? It is pretty normal for siblings to have a major readjustment period.
If you want to feed BM in the bottle then you will need to spend time pumping, which might also take time from DD. Personally I found pumping to be a real pain, it works well for some people, but it was always easier for me to just plug baby in. Can DD sit with you while you're feeding? You could read to her, do a puzzle, sit with her while she draws, whatever it is she likes doing. Almost make feeding time a special time for her.
I know this is very new to you, and so many congratulations on getting this far!, but it will be possible soon for you to plug baby in and sort of act like he isn't there, yk? Try re-focusing your attention on DD as much as you can, tempting as it is to gaze dreamily at DS! She might like to play at feeding one of her dolls or teddies (super cute!).
Otherwise, it is entirely your decision if you want to mixed feed. I see you're already giving formula at night, so you might want to give one bottle during the day. Be careful about timing, you want to avoid too long a space between breastfeeds as it might undermine supply a bit/leave you feeling engorged.
If it were me, I would try to combine BFing and being specially attentive to DD, and see how you go.