Spawn mobs not in your mod?

I made 2 custom monsters in 2 different java forge mods & wonder if there is a way to make one monster spawn the other monster.   ie Say I have a custom mob RedSpider saved in a forge java mod called one:Redspider  & I have a mob BlackCreeper saved in a forge java mod called two:BlackCreeper.       Can I wright a way for the spider to spawn the creeper after the spider is killed if they are not in the same mod?   I like to keep different themes each in a different mod file but there are some mobs that I want to spawn as a stage 1, 2, 3 boss fight but would I then need each stage, items, mobs & biomes & such to all be in the same mod file or is there any way to interconnect 2 mod files?  The fight would spawn each mob Lv1 & then Lv2 after you kill Lv1 & Lv3 after you kill Lv2 & so on but each monster/mob is in a different mod file. Would this be impossible?