MREInfo.com

You should be able to heat two items at once in a Flameless Ration heater. But what happens when you have an entree and a side dish/drink with the heater element sandwiched between is that the water pools under the bottom item and the heater "bars" don't get wetted. Often, only the bottom bar gets activated. Result: cold food.

So here's the trick to getting a hot meal and a hot drink. Instead of laying the boxed heater down flat as usual, prop it up on its side. The water will now run down the side of the FRH, getting to all four heater elements. Tip the box back and forth two or three times initially to help distribute the water, then turn the other edge upwards and tip again. Prop with either edge down.

Wha La!

I do one inside ,one outside,the heating bag, then both in to the entree box. Then lean it on a rock or something almost flat. Works pretty well

I do it like NNF says too - always seems to work for me and eliminates the water-coverage issue of trying to jam two items in the FRH packet

I've recently gone a step beyond that. I no longer put anything inside the FRH bag. Now, I sandwich the FRH between the entree and side/beverage bag. All three then go inside the old box or the newer sleeve. I find that the foods actually get hotter because the FRH works better this way. AND, there's no chemical crud on the heated food pouches, either. I even wrap the whole works in a towel or such for extra insulation if it's at all chilly outside. As a minimum, the outer MRE bag is at least some help; just slide the activated package in (you do have to empty out all the other items, of course.)

One last thing: the new sleeves allow you to slide the entree/FRH/side item sandwich in BEFORE you add the water. The fill lines are still visible at the bottom, and you don't have to handle a rapidly heating FRH.

I totally dis-remember if I have actually posted this in another thread, but this is my solution for boosting the power of the FRHs.
A used Tyvek mailer, lined with the foil mylar bubble wrap used in housebuilding. Lightweight, relatively sturdy, and it seems to increase the efficiency of the FRH's considerably.
Like RockyRaab, I only put the water in the FRH bag and slip my MRE pouches in on either side.