Sarries director of rugby McCall could find no other way of separating Neil de Kock and Richard Wigglesworth, who have been rotated in the position all season.
Wigglesworth started the semi-final victory over Gloucester as part of that policy but Saturday's final is being treated differently and both players agreed to the coin toss.
Saracens will not reveal who won until their team announcement on Friday but McCall confirmed the player who starts on the bench is guaranteed to come on after 50 minutes.
"For the majority of the season we try and run a rotation system, we try and mix it up and keep the squad as fresh as we can for this time of the year," said McCall.
"The rotation system naturally narrows down at this time of the year but there are still some very difficult decisions, for example in the scrum-half position we have two guys who have been rotated completely evenly all season.
"Having rotated we are now not going to choose a favourite for the final.
"For the final we have tossed toss a coin to see whether Neil de Kock or Richard Wigglesworth starts. After 50 minutes the other one will come on regardless.
"The two guys were very sensible. This is the way they want it as well. They will both be involved."
Saracens signed Wigglesworth from Sale last summer specifically because he brought the same qualities as De Kock, allowing a seamless transition.
"We are very fortunate to have two world-class scrum halves," said coach Andy Farrell.
"We have given them both equal opportunities and the best thing about them is that they don't compete against each other. They talk about what they are going to do and the team flows from that."
Brendan Venter, now the Saracens technical director, will be involved this week and he will attend the final - unlike 12 months ago when he was banned following an incident in the stands during a game against Leicester.
Saracens chief executive Ed Griffiths revealed the club requested Leicester write a letter to the Rugby Football Union supporting the idea that Venter, in his then role as director of rugby, should be allowed to attend the final.
Griffiths said Leicester declined the opportunity - but he added Saracens would have written in support of Richard Cockerill had the Tigers boss found himself banned from this year's decider.
Cockerill's behaviour in the stands during Leicester's semi-final win against Northampton was being investigated by the RFU's disciplinary office. He escaped sanction due to a lack of evidence.
"When Brendan was being threatened with not attending the final, we asked Leicester if they would write a letter suggesting he should attend because the final wouldn't be right unless both coaches were present," said Griffiths.
"They were unable to write that letter. I don't know whether it would have made any difference but I just feel these finals should take place with both coaches present.
"It is a showpiece occasion. As (former West Ham manager) John Lyall said of the 1980 FA Cup final 'it's a champagne day'.
"If the roles had been reversed we would certainly have written a letter because we are very happy Richard Cockerill will be at Twickenham."
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