Hi Jason, Yes it’s possible to play the FMB roll with Index - Hammer, index-Hammer, many people play the lick that way. But it’s widely recorded that Earl Scruggs played it as Index-Hammer Thumb-Hammer the thumb coming down to strike the second string and giving more emphasis to note. IMTMTIMT 12125215
Does it save time ? No not really, Smoother nah not really if anything it’s a little more jerky using the index twice at least to my ear. As a beginner this is perhaps the most difficult roll pattern to play. Take your time learn to do it properly with the thumb the way Earl did it. In the long term you will be glad you spent the time working on it. If you set your mind to it you should nail it in a week easy.
Yes there are times you might want to play Index - Hammer, index-Hammer but not on FMB. Ben is a great teacher listen to his wisdom and you won’t go far wrong. Check out Ben’s Video lesson https://banjobenclark.com/lessons/the-foggy-mountain-lick-banjo?from_track=beginner-banjo