i cross trained with ITF taekwon-do last time, but had to quit due to injury. the frequency is twice a week, two hrs training each, does it help? technically/phisically yes, because i gain some aikido insight from another practitioner's perspective. I can apply some aikido technique like tai-sabaki during TKD sparring too. philosophically/mentally no, because being a fighting art it somehow disturb me when i tried to relate aikido and TKD mentality. i'm still eager to cross train, though. i'm looking for some iaido class now, don't know if there's any in indonesia....
anyway IMO, any art you are cross train with make sure that it doesn't contradict very much like the one in my case. look for the one with similar nature/philosophy like aikido-jujutsu/judo, aikido-iaido/kenjutsu, karate/tkd-muay thai, aikido-tai chi, aikido-wing-chun etc.