I have at times, trained with 9 days rest in between each muscle group bro, with quality results. I was doing 10 sets per muscle group at he time, if I recall correctly. I switched between heavy straightbar work on one routine, and heavy dbell or machine work the next time I trained that group. About 6 of those 10 sets were "to failure". Before I started using this routine, I did the twice every 7 days thing, and had more plateus than anyone on the planet, it seemed. After the switch, I immediately noticed strength, growth, and endurance that I thought at times I'd never see. For me, I need the rest, depending on the type of trianing I am doing. I cannot train heavy 2X every 7 days and expect to achieve much progress. Everyone is different though.
The point is, is that you should at least try it, if you feel a switch is in order. Good luck brother