Currently for my group, what i'm doing 85% of the time is what jheusse said (run through the mob until the alts are hitting the mob, then i turn around and attack with my main) but there are some mobs that like to dance around me endlessly for some reason (not sure if it's agro bounce or network latency or something. but happens pretty consistently with certain mobs like the ogres in burning steppes) what i do is i have all the alts movement key bound on the small directional keypad underneath the island of home/end/insert/delete keys, and since all 3 alts are bunched up together from the autofollow, i can effectively control them as one toon in terms of movement most of the time. In those instances i will manually control and walk/turn the alts to make up for the mob moving around me) i'd imagine this method would probably be a lot easier when you're only 2 boxing melees as long as you can get used to controlling 2 toons simultaneously with your left and right hand (main with asdf, alt with directional keypad)

alternatively, what i do sometimes is have alts on autofollow, have main hit the mob but if alts are still out of range due to network lag or whatever, then i'll tap the walk back key for the alts so they stay in place and then i'll manually walk the mob back to within range of the alts melee range. up to now it has worked ok for me most of the time.

good luck! running a bunch of melees is a pain. just hoping pallies get buffed enough in wotlk to make the effort worthwhile.