Hardcore diet

This past week, Logan and Taylor had their 9-month appointment. Unforunately, we found out that they are both falling off the charts.

Logan's height: 27-1/4" (17%)
Weight: 17 lb 4 oz (6%)
HC: 45 cm (40%)

Taylor's height: 24-3/4" (7.43%)
Weight: 15 lb 7 oz (0.31%)
HC: 43.5 cm (8%)

These last couple of months, I've been feeding Logan and Taylor solids 2 times a day (at noon and in the afternoon) and 2 bottles a day (in the morning and at night). Since this obviously hasn't helped them to gain weight, our doctor suggested feeding them formula before solids, since formula has more calories than most solids. She also suggested feeding them 5-6 times a day instead of 4.
A couple of weeks ago, I had tried to feed Logan and Taylor more often, every 3 hours, but those 2 days were absolutely miserable. I ended up stressed out and very frustrated. But the difference was that I had tried this with solids, when I should have tried it with formula.
So for the last couple of days, I've been trying to stretch their tummies by giving them 5 bottles a day. Sometimes they can finish a whole bottle in 15 minutes, and other times it takes us 45 minutes (EACH). I also give them solids twice a day, after a bottle, but they are usually pretty stuffed by the time we get to solids, so they usually don't have anymore than 4 oz.
Taylor is supposed to have 680 calories a day, and Logan is supposed to have 790 calories. Most days they do well, occasionally they are a little under, and sometimes they are slightly over. I really really really hope that this new diet will make a difference and that they will gain weight. I am afraid that if they don't, their brains won't get the calories that they need, and Logan and Taylor will not develop at the best rate. I hope this works... I pray this works...