If you default on those two cards, those creditors will report the lates/defaults. The creditor with the card you are keeping will notice.
They may take adverse action or they may not. Much depends on the creditor and your complete credit picture.

If they do take adverse action it could be in the form of higher interest, decreasing your current credit limit or in extreme cases it could be account closure.

If they do take adverse action and raise your interest or decrease your credit limit, you may have to live with it for around 6 months, or so, until you have proven to them that you are continuing to take care of that particular credit card. After 6 months you can try to get them to lower your interest and/or raise your credit limit.

If you are planning to file bankruptcy, you may be able to keep that one card but it will have to be at a zero balance when you file. Plus if you are carrying a rather large balance and pay it off right before you file, that payment can be recalled by the bankruptcy court and you may end up having to include the card.

If you are planning to file, you might keep that card as close to a zero balance as you can and only use it every couple of months for very minor purchases and pay in full when you receive the statement.