Nope they wouldn't be able to defeat him...
Madara was able to break the same seal used by Hagoromo and Hamura in his old age so I don't think the seals would work on him in his strongest form.
The reason they were able to seal the limbo clone was because Madara transferred the effects of the attack used on him to the limbo clone.
They both tried to kill Madara the whole time while Madara wasn't completely serious, so I Don't think they could do anything after he became completely serious.
Sasuke made it clear that they need to wait for a chance or they would both die
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the manga made it clear they would both be dead if it wasn't for BZ.
edit: Madara has 5 limbo clones and the limbos can make more limbo with all of them using PS powered by Hagormo's yin and yang chakra, the juubi's chakra and Kaguya's chkra that he got when awakening the rinne sharingan, so no they don't stand a chance and that is why Hagoromo gave Naruto and Sasuke the seals and his chakra to seal a mortal Madara not the immortal version.