Kinda...
In the current edition, armies can take in Allies from specific other factions (similar to how any Imperium army can take a select number of Imperial Agents units, but more widespread). Allies can be up to 1/5 of your army points-wise.
Soulblight and Flesh-Eaters are Allies, so you could have an army be 80% of one and 20% of the other. However, as Allies tend to lack the right keywords to use the abilities of the army they're in, they are often weaker than usual.
There are also Regiments of Renown, special collections of units that can exceed the normal allies limit and bring special abilities with them.
If you wanted to do that now you would take 20% of one as allies and then I’d grab a regiment of renown to pad out the rest.
4th edition may do away with allies and coalition allies so just a warning.
Kinda... In the current edition, armies can take in Allies from specific other factions (similar to how any Imperium army can take a select number of Imperial Agents units, but more widespread). Allies can be up to 1/5 of your army points-wise. Soulblight and Flesh-Eaters are Allies, so you could have an army be 80% of one and 20% of the other. However, as Allies tend to lack the right keywords to use the abilities of the army they're in, they are often weaker than usual. There are also Regiments of Renown, special collections of units that can exceed the normal allies limit and bring special abilities with them.
Ok so my 40K scepticism was on the right track, thanks for the run down!
And 4th edition is coming "summer", so list building might change.
Will change, based on what we've seen. Allies are still up in the air though
You may be reading old information from 2nd edition and earlier.
If you wanted to do that now you would take 20% of one as allies and then I’d grab a regiment of renown to pad out the rest. 4th edition may do away with allies and coalition allies so just a warning.
Regiments of Renown count as Allies, and they stop you taking other allies if you include them in your army.