Your characters are good in team, but can't go really far without a support unfortunatly. So I would either recommand you to change characters, or to play on bnet with other players.
If you decide to change character and never play in team, but gear yourself easily, you could chose some really good solo character. Crusader and DK are best for that. They have decent damage and huge self-heal.
If you decide to change character and never play in team, but great challenge, you should chose some hard char to master without be insane at solo. For example Witch and Alchemist should probably be able to duo rectus and FN with specific gear and much skill. (ofc I'm excluding FN+ gear for that, full alch can probably solo but it's not the point)
If you prefer get some gear solo and then try go in team, try to get some different kind of character. It's easier to include in a team 1 thunderer and one WiM more than 2 th or 2 wim. But in that case I would recommand to both chose the character you like the most, stop whenever it gets impossible to get more gear duo with these characters and join a team whenever.
About difficulty I volunteerly chosed 1 very easy and 1 very hard, tbh I doubt you manage to do something with the very hard (on FN and rectus), but I tend to underestimate these 2 char so maybe. You have many choice and there isn't really some character you have to avoid in duo. (MA SW should be in DUO)
But get 2 dps that can't heal won't make you go really far, DL/SQ probably limite, maybe MC if you manage it very well with some good gear. (And would most likely include you farming spectres to get death's visage without kill MC itself first)
About boss impossible to duo (without get some insane endgame gear), there is SB TL AE SK SM ZL SD. Though with endgame gear SB has been soloed (by me) with dark knight, and I heard a legend of a korean soloing TL with alchemist. I would forget try any kind of "duo kill" of these as long as you don't have SKAEZL(SM) gear.
About SB, if you are REALLY patient, you can possibly get SO from mana ancient, and hopefully you won't need any of his items.
I think I said everything, gib more info and I might tell you more of what you could do. But I personally recommend to play your favorite character each and join bnet players whenever you feel you can't grind more duo.