One player playing archer and the other playing cav is in general the meta in 2v2.
During the feudal age it is more likely 1v1 and you start playing with your mate in castle age or late feudal.
Spanish is not a really good civ on open maps unless you can wall and go for conqs on castle age.
Conq are known to be probably the best unique unit in castle age and can be played to apply a lot of pressure on your ennemies.
Mongols can play both cav and archer, have access to camel and mangudai (amazing unique unit you should go in imperial age).
One reasonable strategy for you would be to go scouts into conqs and archers for your mate.
But you should first have fun and enjoy your games!