Goalkeeper Collis and right-back Ofori-Twumasi have joined the club until the end of the season and are both in contention to make their debuts at leaders Chesterfield on Tuesday night.
Johnson said: "We need a second senior goalkeeper, someone who can compete with Chris Dunn.
"Steve is a person I know well, and someone who has been with me before. He plays well, he trains well, he has a great attitude and is a player I can trust.
"I have been impressed with Chris Dunn, but Chris needs competition and needs to battle with Steve for a place in the team. We also have to consider the loan window is closing soon and we need to have more than one senior keeper in case of injury or illness.
"Nana can play as a right-back or centre-half and he can also cover in midfield. He has got power and pace and we have brought him in with a view to making the move permanent if everything goes well. We need more mobility and power in the squad and more pace."
Source: PA
Source: PA