you will still find /follow drops occasionally even on 2 PC's. not as often, but it does happen when youre lagging or going through a load area (or your character gets stuck on something).

its much worse running more than one client on one PC, though. i find it helps if you start off moving backwars or walking to get your clients moving and then speed up.. the reason is, if you start off at full speed and theres enough of a delay between you moving and the server sending the follow command to your alts.. you will be out of follow range and they will only take a step or two and then drop /follow. happens a lot on horse.

turn around, move backwards till your clones start moving a bit and then 180 and you should be fine at full speed.. works for me.

also helps to have /follow MAIN macro bound to somewhere convenient and get in the habbit of clicking it a lot.. even when its not really necessary. i click it every few seconds when im engaged and running around spamming nova/arcane explosion.. i rarely use any non-instant spells in PvP (dont like standing still) so clicking the follow macro gives my clones a bump and makes sure theyre still tagging along =)

nothing worse than backing out of a PvP engagement, in battlegrounds, for example.. and then realising youve accidentally left one of your toons behind =(