merging two forest

hi, am currently looking up to merge a.com and b.com the final outcome would be c.com.
i have a domain controller and an exchange server from each side which needs to be merged with the new c.com. i cannot bring over 48hrs of down time to the networks.
for creating c.com i have two extra servers which i am planning to use one for the DC and other for the exchange of c.com.
through reading i found that i could do a forest trust and continue. but am not sure so nyone pls do guide me or least show me the rite materials to read.

thank you