My newest mobo is very finicky about this(Gigabyte H97-something). If they're even same brand but different speed, it's a no go. Newest BIOS & all other tricks were tried. Ended up selling memory at a loss to replace with 2 x 8GB. Lost almost as much value as the mobo cost in the 1st place...LOL.
This is the 1st time I've ever had problems running Dual Channel with mixed sets before in many years of PCs. So, I guess, in answer to your question, depends on the mobo.