Let's say in my army list I have 2 units of 5 Furies, and in my first game I play it as the list is written, 2 separate units.

Against my next opponenet I feel it would be far more advantageous to use 1 unit of 10 Furies, is this legal?

I mean my army is the same as my list, and I haven't changed any points or equipment obviously, just combined the 2 units. I'm afraid people are going to say I can't do this, but it seems logical to me to be able to do it, to me it's basically just deploying them differently, and would seem silly if I couldn't choose to combine them. As a general commanding 10 Furies I would think I could deploy them any way that I want as long as its legal and I'm not altering the points or equipment in my list. I need this answer because I'm thinking of bringing Furies to my next tourney if I can do this.

I'd say it's not legal unless the tournament organiser agrees with you. Ask them beforehand.

You'd think you could do that but I've never seen a tournament that allows this. You play with what you took, there's really no difference conceptually between doing this and playing a completely different army. Consider that 2 units of Furies are 2 slots from the armylist whereas 1 unit is only 1 slot. Comparing this to changing the whole army is only a matter of degrees.

At most you can choose a different lore for your wizards, as that is picked on deployment.

I'm going to assume you are playing Deamons of Chaos?

Not

I do not think this would be legal. You play with exactly what you have on your army list.

Yup, its not legal. You have to make the decision about which size of unit is more advantageous to you before the tournie.

Its all part of designing your army, making the hard decisions.

Which armylist are you playing with?

Pick one, because you only get to take one.