While both have implied they'd also refuse to abstain to let one pass, as far as I know, neither have come out against it.
To me this indicates the strategy on the part of both men.
If Harper wins the most seats, or even ties with someone, they will allow him to continue... up to the speech.
During that time I expect both (Trudeau and Mulcair) would talk to one another. I expect Mulcair to offer a coalition and Trudeau to turn it down. Regardless, they will feel out what the other would do in government and then find some way they can support one another without it being political suicide. I expect a deal, either for 18, 24, or 36 months, that will see one of them form the government with a level of support from the other. I doubt it would be a full coalition (with a cabinet made of members of both parties) but may be a half coalition (with the budget written, in some part, by both parties, but only one party in cabinet).
Feel free to share your thoughts on this below.