Liberal Democrat Sir Bob Russell and Conservative Sir Edward Leigh said the Lords Spiritual (Women) Bill was an opportunity to increase religious representation in the Lords.
The bill was drafted with just one purpose because it had to be passed quickly, said constitution minister Sam Gyimah during a debate on the issue.
If passed the law would mean female bishops would skip the queue of their male colleagues who are waiting for a seat.
The debate was watched by the Archbishop of Canterbury Justin Welby from the gallery overlooking the Commons.
Sir Bob said: "I'm sorry to bring a note of discord to the proceedings but I am bound to observe this whole arrangement is for one denomination of the Christian church.
"Surely in the 21st century, if we are to have religious people in the Lords they should be more representative and reflective of all Christian denominations and indeed all faiths."
Sir Edward said: "Tomorrow, we are celebrating the 750th anniversary of our first parliament ... of course, then every bishop would have been Catholic.
"All I am asking ... bearing in mind women priests unfortunately, sadly, does make union between our two churches less likely - does the Government have an open mind about allowing bishops of another denomination to enter the House of Lords?"
But constitution minister Sam Gyimah said the bill was not about that: "This is a very focused Bill, it does not really seek to change the composition of the House of Lords or the powers of the House of Lords - what it is seeking to do is allow something which would happen anyway, the admission of female bishops into the House of the Lords, to happen sooner than it otherwise would."
"This is a modest but important Bill.
"It has one simple aim which is to bring female bishops amongst the Lords Spiritual sooner rather than later. Given how long women have waited to become bishops that is right.
"The House of Lords should not have to wait for an unknowable period of time before its Lords Spiritual benches reflect the new make-up of the episcopate."
Labour's constitutional reform minister Stephen Twigg backed the Bill, saying it was an "important symbolic moment".
He said: "Today we can all come together to recognise what could be a momentous moment for our church and our Parliament. Another step towards true gender equality.
"We know there is a long way to go. As (Mr Gyimah) said, female priests were introduced 20 years ago and out of 8,000 full-time priests in this country, 1,700 are now women.
"In the original draft of my speech I pointed to the slow progress that represented, but on reflection when I worked out the percentages I think it's about the same proportion of women priests in the Church of England as we have women Members of Parliament in this House.
"In other words, the Church of England has achieved in 20 years what we've achieved in over 100 years so it's very significant progress nonetheless and today's Bill is an opportunity to build on that progress."