Moving up to 10/20 or adding another 5/10 table?

[chessmaster]

Im quite new at SH and played some 5/10 this month strictly one-tabling because im not too good at handling multiple SH tables (I seem to play looser with more tables and my laptop-monitor has only a resolution of 1024*768 so i have always overlap).
I did reasonably well so far being at about 4BB/100 after
5k hands with playing mostly tight/passive (my postflop AF worries me a bit, is only around 1) but i catch so many bluffs that im still profitable.
Id like to either add a second table of 5/10 or move up
to 10/20, so i tested yesterday first twotabling 5/10 with very bad results (-50BB in 2h) and then today onetabling 10/20 with good results (+40BB in about 5h).
I now wonder what would be best to do next, learn multitabling at 5/10 (and maybe buy a new computer and monitor to avoid overlap) because i have to learn it sometimes anyway or just move up to 10/20 because the games seem to be even looser than 5/10 (bluffraising the turn with just overcards seems to be standard play of some people there). I think im not ready for twotabling 10/20 so far because this could turn out really bad for my br.
Or do you think i should first get a bigger sample-size with onetabling 5/10?
Any suggestions?