"In the next two weeks" vs. "next two weeks"

Promoting a comment to an answer:

Are your friends planning a single trip to the beach or plan to spend one week or two weeks beginning in 14 days?

In the mean time I would suggest any of these:

We have decided to go to the beach

• in two weeks' time - trip starts in approximately 14 days and can be 1 day
• two weeks from now - trip starts on this weekday 14 days from now and can be 1 day
• the week after next - trip starts in 8-14 days and sounds to me it will last a week

"My friend and I decided to go to the beach in the next two weeks." This is a grammatical sentence, but it means that my friend and I decided to go to the beach some time during the next two weeks. If you want to refer to the week after next, then you should say "My friend and I decided to go to the beach the week after next".

This answer accepts, corrects and expands on the others. Here is a fuller list of correct ways to say it:

• My friend and I decided to go to the beach the week after next.
• My friend and I decided to go to the beach in two weeks' time. (An apostrophe is required here, though often people forget it.)
• My friend and I decided to go to the beach two weeks from now.
• My friend and I decided to go to the beach in two weeks.