The Resident Evil:DBG rules don't specify because you are not supposed to combine the other cards, just the mansion cards.
Advanced Note 5: The Day-Killer - Feel like Resident Evil™ action all day long? Combine the Mansions from Set 1 and 2 and rejoice in the Infected hunting fun! The game ends when both “Albert Wesker” and “Uroboros Aheri” have been defeated.
It is your game, so you can play it however you like. Officially though, you are only supposed to combine the Mansion Cards for the Day-Killer variant.
As for what effect this might have, and if it would be any unbalancing, it depends.
A common tactic in a four player version of the game it to buy up the Ammo x30 (and sometimes Ammo x20) cards before your opponents have the ability to draw into a hand with enough cash to buy those cards themselves. This will deny them from buying the really expensive weapons in a particular 18 pile distribution. If you double the Ammo cards from fifteen to thirty copies, this tactic probably won't be possible.
Normally, there is only one Burst-Fire Handgun, and one Survival Knife. Players with additional buys have a greater chance of picking up those weapons for their own use, or to deny them from other players that might benefit from them more (i.e. Jack Krauser). Having multiple copies of the Survival Knife in the Basic Resources makes it harder to deny Jack Krauser at least one Survival Knife. If Jack happens to get two copies of the knife, things could get even worse. This is of course somewhat balanced by the fact that there are now twice as many regular knifes that Jack has to cycle through before he gets to the special one.
The other Basic Resources Green Herb and Ammo x10 will have little effect on the game when doubling their number.