You can halve the price to a fiver each with Groupsave for a start. Possibly easier than faffing around with advance tickets.
There are no
Advance tickets on the flow Watford Junction - Wembley Central.
Ignoring Oyster PAYG, the fare is £10.00 for an Anytime Day Return. Off Peak
paper tickets are not available for this flow because they want you to use Oyster PAYG instead.
However from Watford North the
Off Peak Day fare is only £5.40, and it is permitted to start 'short'. London Midland have a half price sale on, so that reduces the price to £2.70 each, totalling £10.80 for 4 people. The tickets would need to be purchased in advance, but they are not Advance tickets, they are Off Peak tickets.
Alternatively
GroupSave may be an option, this would be £20.00 for 4 people from Watford Junction [[stn]WFJ[/stn]], or £10.80 for 4 people from Watford North [[stn]WFN[/stn]], but the group would have to stick together.
I would take advantage of the LM half price sale from Watford North and simply start short at Watford Junction.
http://www.londonmidland.com/tickets-and-fares/half-price-summer-rail-sale/