They are almost same. I think... 「込み合う（混み合う）」 seems more crowded than 「込む（混む）」　. I think, "込み合う" often has a nuance, "crowded and crowded". Though, the difference seems little. p.s. 「こむ」 can be written, 「込む」 and 「混む」.

we japanese often use "こむ" with other verbs. like the following example. 彼は黙りこんだ : he shut his mouth and wouldn't say a word. 今夜は冷え込みそうだね : it's going to be really cold tonight, isn't it? 私は娘に料理をきっちり教え込む : i teach my daughter cooking and let her master it maybe my translation is bad. but hopefully it was helpful to you.

No difference. I would just like to point out that wrong kanji is used here. "混む" and "混み合う" are proper.