If you change feeds etc. Too fast or too often this can cause a lot of stress on the horse and even colic. You can't make a horse gain a lot of weight in such a short amount of time.
High fat in the yellow bag will be you best bet , but it’s not going g to change in 2 days
I don't think that it will be possible in two days. To fatten up a horse, you must take your time, if not, you will probably cause health problems to the horse.
It's not possible in two days