Adwords Budget Setting Question

Quick question for any adsense gurus that might be lurking around - is it possible to set an individual budget for an ad group within a campaign?

So lets say I have 'Campaign 1' that has a max daily budget of £100 and there are two add groups ('Ad Group 1' and 'Ad Group 2'), can I set it somehow so that both ad groups have a budget of £50 each?

Obviously the easiest way to do this is to create seperate campaign, but I want to have some cross over in case one of the ad groups has space left in its budget but the other runs out early, whilst still keeping within the main campaign budget. For example:

Campaign 1: £100/day
Ad Group 1: £75/day
Ad Group 2: £75/day

Here if ad group 1 only got £25 worth of clicks, that would allow ad group 2 to go up to £75.

Haven't been able to figure it out, so I'm guessing you can't do this - but I thought it couldn't hurt to ask!