I wouldn't use a mouse button as a modifier. WoW can be flaky about mouse 4/5 as it stands and it's usually suggested that mouse 4 and 5 be changed to a standard keyboard key instead of a mouse button. I would suggest going into your mouse software and configure those keys to something else.

http://us.battle.net/wow/en/forum/topic/7923843957
http://us.battle.net/wow/en/forum/topic/5762727758
http://us.battle.net/wow/en/forum/topic/2030063919
http://us.battle.net/wow/en/forum/topic/1869564497
http://us.battle.net/wow/en/forum/topic/6079779411

The next problem I see is you are using <SendWinM WoW1> and <SendWinM WoW2>. SendWinM should not be used within the hotkeys, it's only used until the windows have been assigned a label. All hotkeys should be using SendLabel as I show in my posts in the thread you linked.

The AoE button I use is modified down to a single press and release

Code:
//* 1-BUTTON AOE
<Hotkey NumpadPlus>
	<If ActiveWinIs WoW1>
		<SendLabel w1, w3, w4, w5>
				<Key Oem3>
				<Key F9>
		<SendLabel w2> 
				<DoHotkey Hotkey Alt F9>



<Hotkeyup NumpadPlus>
	<If ActiveWinIs WoW1>
	<SaveMousePos>
		<SendLabel w3, w4>
			<Clickmouse LButton>
			<RestoreMousePos>
		<SendLabel w1, w2, w5>
				<DoHotkey Hotkey Alt F9>
First, it only works if the active window is WoW1 - my main. It's not usually an issue, but if you spam the key too much, HKN can get stuck on a slave window, which means the saved mouse position that HKN tries to restore to is going to be the wrong window. Allowing the key to trigger only if WoW1 is the active window circumvents that issue.

Second note that it is an unmodified key "NumpadPlus". This is actually a remapped key on my mouse {then it was a Razer Naga, now it's a G600} that corresponds with the last key on my mouse's thumbpad {Naga = 12/G600 = G20}. I tend to prefer to use unmodified keys as often as possible for combat actions, just because there's less of a chance that the modifier gets lost, stuck or missed and there's plenty of keys I can't generally reach that I can remap on my mouse. I still use plenty of modified actions, but I try and restrict it to stuff that isn't that time-sensitive.

Third, note that everything uses SendLabel. This is how you are supposed to send keys to each window.

As far as the action itself, you will see it's 2 pieces of code, Hotkey NumpadPlus and HotkeyUp NumpadPlus.

The first part, Hotkey NumpadPlus, is the spell trigger. In my case there are 5 toons, 4 of which have damaging AOE that would be called {w1, w3, w4, w5}. w2 is my healer and she has no AOE to use, so she just does her normal spells {<DoHotkey Hotkey Alt F9>}. The other windows receive 2 keys - Oem3 which is my follow button so my slaves all stand near my main, and F9 which is the actual AoE spell. So when I press my "AoE Key" down, my melee slaves {w3, w4, w5} follow my main and trigger their AoE spell.

The second part, HotkeyUp NumpadPlus, places the mouse click to drop the targeting reticule. 2 of my toons {w1 and w5} are paladins and their AoE doesn't use a reticule, so they don't need the clickmouse, so those windows now join w2 in doing their normal spells. w3 and w4 both require the mouseclick, so they get the clickmouse command when I release the AoE key.

If you just want a simple version for 2 toons, all you need is this

Code:
<Hotkey NumpadPlus>
	<If ActiveWinIs WoW1>
		<SendLabel w1, w2>
				<Key F9>


<Hotkeyup NumpadPlus>
	<If ActiveWinIs WoW1>
	<SaveMousePos>
		<SendLabel w1, w2>
			<Clickmouse LButton>
			<RestoreMousePos>
Of course you can change the "Hotkey", "Hotkeyup" and "Key" to whatever you want to use. Hotkey and Hotkeyup are the actual button you press on your mouse. Key is the button your spell is on in game. They can all be the same to make it less confusing for you.

So, just to reiterate on how to use it -

1 - Press the key down. This triggers the spell in game.
2 - Position your mouse. You can do that first, instead. This step is just to show you that as long as you are holding the key down, you can make last minute adjustments before placing the targeting reticule down.
3 - Release the key. HotKeyNet will then quickly move from screen to screen clicking the ground where the mouse is located to use the AoE.