As people have said this will depend on the terms of the contract you sign for the Spring TC, or if the Spring firm will extend your offer deadline and the Summer Scheme doesn't prohibit this.
However if we want to be very real here, there are also zero consequences if you were to do this anyway, even if you are not allowed. Not saying that you should, but saying A LOT of people do every year. I believe the SRA used to have a stipulation that they frowned upon this, though they removed it a few years ago. On my scheme last year I knew of two people who had already secured TCs.
It only gets tricky if there's a financial commitment from the firm you've accepted a TC with, or if the Summer Scheme publicises their scheme. By this I mean, if the Spring firm has already paid for you to start to the GDL or SQE or whatever by the time you're doing the Summer Scheme, you may be financially tied and may have to reimburse them should you withdraw from their TC (though this is not likely in June or July). Also should they discover you are on another scheme (through linkedin/instagram etc), that they forbid you from going on, they can rescind your offer. Then if you don't convert the summer scheme you are left without a TC.
Ultimately, it is up to you. Some people still frown upon it, say it's ethically dubious. Some people say it's a competitive market and that firms restricting candidates like this would be unfair. Use your own judgement.
