"discuss it beforehand" is correct and usable in written English.
It is generally used when talking about two or more people making an agreement to have a conversation before making a decision. For example, "We need to discuss it beforehand to ensure we are making the best decision."
"We didn't discuss it beforehand in a lot of details," said Ms. Hauner, who, like Mr. Voss, is a postdoctoral neuroscience researcher in the Chicago area.
S.F.J.: How do you assemble set lists (if you do)? S.A.: We played quite a few songs that don't appear on our albums, but we didn't discuss it beforehand.
Discuss it beforehand and always, always use a condom.
The gift, apparently, was her idea — they discussed it beforehand.
The incident today appeared to share one fundamental common trait with other similar shootings by teenagers around the country: the gunmen discussed it beforehand.
One person involved in the inquiry said that the senator and his former wife said Mr. Chang gave the gift without discussing it beforehand with Mr. Torricelli.
Frankie [Dettori] and I discussed it beforehand and we decided that we were going to follow Jet Setting, as she was sure to go up the near rail and, indeed, she did go up there.
Mr Osborne told the inquiry: "I was aware and we'd discussed it beforehand internally that hiring him would attract some controversy.
In an interview, Mr. Buechner characterized his dismissal as "less than gracious," saying Mr. McCain did not call beforehand to discuss it.
It requires approval from a high-level officer — in this case, an assistant chief who also discussed it with Beck beforehand.
Host Stephen Colbert autographed her thigh, an idea by Reutter that initially caught the comedian off guard (Although Reutter's website suggest that it was her manager's idea and they had discussed it with Colbert beforehand).
