I've done 1 on 1 sessions... and they can be great for getting plot done with out it being the whole groups burden of player knowlege,,, as for the 2 on 2... very do-able... remember your are not just the DM your a player too.... The DM runs all the worlds NPCs... that is known... but try making a DMPC i like to call them... they are you personal character... to round out the group and help the players in the campaign...
Be sure to build the character just as they would build theirs with them would be even better... If you the kind of DM that starts adventures... right at the point of the group being together and sort of having them already have background with eachohter this is perfect for this example: Jack (PC) you and Tully(PC) have been traveling for days... you finally reach a small town... called Ashmoon after getting a bed for the night and resting you make your way down to the inn... there you see a familar face... it's Gnarl "DMPC) the handy dwarven smith... he's done some side jobs with you before... he can always be trust, plus there isn't a lock made he can't open... at least that's what he tells you all the time.
This way it;s a lot like 3 people play... I also think they idea of encouraging duel class... if you feel it;s needed.. but there is nothing better then relying on tact and preperation too win the day... hope this helps... good luck...
"My kindness is not my weakness, It is your Handicap." Z'izum(C)
"We are the Dreamers of Dreams, We are the Music Makers" W.W. & C.F.(C)
"Mercy is for those who believe they deserve forgiveness, I on the other hand have no such delusions." ~Kern Dragonborn~