really you just have to wait until they trun 3 and apply for the mobility component.
most families with more than two pre-schoolers have a boot full of buggies etc, so you would only really stand a chance of getting charitable funding if there was other specialist eqt that was necessary (eg portable oxygen, or if both children needed much larger sn buggies with a lot of positioning eqt/ head supports etc. standing frames for both children for nurser y - although the easiest option there is to get the lea to provide a second set of frames to ease transportation requirements)
if this is the case, then you could ask your ot or physio which local chairities might support an application, but if they are two already then given the time it gets to get stuff through and approved, it might be easier to wait until they are three and qualify! 
i would def start asking around for ideas as to which suits your needs though. maclaren majors are pretty big (don't know which buggies you are using at the mo, but they seem to be pretty ubiquitous through wheelchair services for children with mobility needs but no support requirements) but probably aren't necessary until about 4, unless your kids are quite big (you can get doubles etc), but if wcs are looking at wheelchairs longer term it might be better to steer away from buggies as early as poss. (ie if the boys are going to be lifelong wc users then there is an argument that says get them in as soon as possible - particularly if they are likely to be able to self propel to encourage independence.)
i know i asked on the other thread - apols, but what dx do they have? it's often tricky to get wcs referrals early, as it's cheaper for the nhs to keep them in regular buggies whilst they still fit. what do your ot and physio say? have you got a wcs referral yet? are they likely to be able to self-propel or are you looking at power chairs alter on? this is the sort of stuff you need to think about longer term in order to decide what sort of vehicle would be appropriate. are you thinking of transfers, or are you likely to be looking for a specialist van with two wc spaces and a ramp? your long term requirements will need to be determined before you think about how much money you will need to apply for etc.
it's tough to do now as presumably although it's unlikely they'll walk, you don't really have a definitive idea of what their capabilities and support requirements will be? i know when dd2 was 2 and not walking we had no idea, so we traded our car for an old knackered ford galaxy so that it plenty of space for the chair and standing and walking frames. it gave us time to work out longer term what sort of vehicle we were likely to need.
when they hit 3, if you look at motability you will be bombarded with choice. but there are still cost decisions to be made. at that point it's easier because you have the official blue badge designation and might have a better idea of the next few years.
in some areas they might consider blue badges prior to 3yo, but i think it's still unusual - if you think you might qualify for special dispensation (again, usually for other specialist equipment or casting that requires space rather than needing a buggy) then get the paed to write in support. just using the bigger car spaces can be really helpful if you are manhandling children with muscle tone issues and are unable to support themselves. it's worth a try!
you would need really specialist requirements to get any sort of funding for a vehicle before 3. the age was chosen specifically as most nt children under 3 still use buggies of some description.