Sometimes you can't have your cake and eat it too. You can always ask if you can wait six months to make a decision, but only if you're willing to have them say no.
Lots of companies take advantage of interns. They use pressure tactics like exploding offers to lock them in to positions which don't even pay market salary for their effort. This is an unfortunate situation, but not uncommon.
At the same time, 6 months is a lot of time. If the company has a lot of interest in their program, they would rather fill the slot sooner rather than later. If they do give you 6 months because you're a superstar, there is the very real possibility that the other company will find you a superstar as well, and you will take the other offer anyway.
In the meantime, they have to find a candidate who will wait 6 months for their decision. Good candidates will be able to find a solid offer, not one that is a 6-month 'maybe' so they will lose other good potential candidates that way. From the company's perspective, 6 months is a huge investment to make in a maybe.
What 6 Months Signals to the Company
When you say, "I would like to wait 6 months to make a decision" you are telling the company:
- Your offer is a backup offer, not my first choice
- I am waiting on better offers, and will take them if I get them
As you can imagine, most companies will not be so content to wait 6 months for a candidate who regards them as a safety net.
Disclaimer: I am not saying that this should be done, only that they are options. Decide what to do based on your own sense of ethics and your own goals, not because someone wrote something on the internet
One thing you can do is to look at the current offer (the contract) and see what it says about withdrawing from the internship. If there are no penalties, then you can sign the contract and if you end up getting the other position, you can break your agreement with the first internship. This may create some bad blood with the first company, but it will give you the most flexibility with your options:
- If you don't get the second offer, you have an internship and nobody is the wiser.
- If you do get the second offer, you can make the decision which to go with.
The other option is to be completely honest with the company. Something like:
I have promised to apply at another company as well, but their evaluation process doesn't start until March. If you can hold the offer for me, that would be great, but if you can't and someone drops out of the process in March, can you give me a call and let me know?
There is a good chance that other interns will also tentatively accept the offer and then turn it down if they get a better offer. By being up front with the company, you may be able to set yourself ahead of the other candidates, and give yourself a better chance of getting a call if they have slots to fill on short notice.
This will give you a bigger risk of ending up with no internships, but it also shouldn't cause bad blood with the company.