Hi Lex. My ds and dd went through the same thing around 4 months, which sounds about right (due to corrected age). Well.. the bad news is, they never tandem nursed well after that. The good news is, it was only a month or two before they were eating less frequently (fewer but longer feedings instead of the on/off 24/7 of newborn days), and I really enjoyed the special time with each of them separately. I found I just liked it better. They certainly did. They got accustomed to their own nursing styles, their own favorite positions, and so on instead of having to fit with their twin. So all in all, it was a good thing for us. But not everyone feels the same way, or has babies who are as opposite as mine - dd likes it very quiet and still, as though it were her way of regrouping and having some soothing R&R. But ds is a very active nurser... he wiggles all around, plays with my hair/buttons/shirt, bobs on and off several times, looks around... etc. etc! So dd would get it his way, and for dd, ds was disturbing her. But again, those are my babies.
Hopefully you will be able to tandem nurse well soon, since it seems like you want to continue that. I'd lose the pillow though. (After 2-3 months, they didn't like how stiff it was.) Do you have a recliner or a big armchair? Try sitting back with some soft pillows under them - football style. That might be more comfy.. but again, that's just what my two liked.