That's because at 1900 the opponents start becoming somewhat decent, everything before that is trash. If you take the rankings of a 1800 2on2 team and compare them to the ranking of a 5on5 team, you'll see that the same ranking in 5on5 has a rating of <1500. From here you need a lot of practice.

The biggest problem War / Pal has is that they have no CC while they are very vulnerable to CC, the Paladin does not have any instant cast abilities that would help here, so just being spammed with short interrupts will keep him from healing long enough for two DDs to kill a warrior.

This is suprisingly quite true. While I joined a 2v2 team a month or so ago as a fill in I hadnt actually played any games in that bracket. I am reasonably happy with my pve geared mid 1600 rating in 5v5 as a reflection of my fairly sad pvp abilities But in 2v2 we only intended to get the minimum ten but stopped after a hour with 15 wins out of 16 games. 14xx to 16xx and it was obvious the difficult was not at all close to a similar rated 5v5