I have done this in the past when going with a friend who supports the home team and I support the away team. You have to resist the temptation to shout 'offside!' or 'handball!' against the home team because you'll find yourself wanting to.
The only time fans around me have found out is when I saw Villa away at Anfield and my uncle is a Liverpool fan, so I sat in the home end with him. We were getting battered 3-0 and just had a laugh and a joke with the home fans around me about how bad my team were that night (in 2010). But it can be dangerous.
Looking forward to winning the league cup in 2020