The Scriptural basis has already been provided above. In addition the following practical reasons are applicable:
1. Safer than only one person (especially a woman--the majority of those in field service) going to a stranger's door.
2. It helps to have another person because if there are points raised or even just additional things to bring to the discussion, two heads are better than one.
3. Actually it is less suspicious for the householder. A householder seeing just one person standing at their door (even if dressed nicely) might reasonably fear a burglar, rapist, etc. (most criminals of this type do their work solo not in twos). So when a person looks out and sees two persons and more on the street and it is obvious they are all dressed alike and of the same group, they can feel at ease and probably know already that it is JWs.
4. For record keeping purposes, it is easier if one of the persons keeps the records, housenumbers, not at homes, placements, etc. and the other can help, than just one person trying to do all of this and think about their presentation also.
5. It is encouraging to have a partner, they can talk, motivate each other to do more, etc. If it were just one person they might get discouraged, bored or tire out early.
Usually preferable to have at least one brother with the group.
Men and women don't work together unless siblings or married.
Groupleaders don't like to let the whole group out of sight and if one pair or person goes into a house --to actually sit and talk then usually this makes the leader very nervous since he doesn't know what is going on.
Paradoxiically, universally what is being offered is a "free home bible study" but few witnesses actually accept an offer to get started immediately!!!! This throws off their time, the group's plans--since the group is reluctant to go on without them or can't if they are driving. Delays them in taking their coffee break :-) etc. etc. so if you wanted to really screw up the whole group invite them in, serve coffee, make some snacks (the witnesses will feel rude and have to accept) and then just talk about all kinds of things or pull out a bible and have a nice discussion.