This page on Single Account vs Multiple Accounts will discuss the advantages and disadvantages of having a single account vs having multiple accounts of My Singing Monsters and My Singing Monsters: Dawn of Fire.
Usually, having a single account will allow less scatter of supports (such as lighting Wishing Torches and providing Keys) by friends; however, having multiple accounts can allow more "friends" to add friend-based supports. However to that however though, it is a lot more expensive to grab multiple accounts with multiple devices, particularly with getting the iOS-based mobile devices for supporting the multiple accounts to be supporting your main account. It is also more time-consuming to try to update and play every single account of My Singing Monsters and Dawn of Fire to a reasonable standard.
Advantages of Multiple Accounts/Disadvantages of Single Accounts
The advantage of having multiple accounts is that you can help your own main account, such as adding multiple torch lightings to your main account's islands or even using every one of your accounts to light every other accounts' torches. You can even recruit your own multiple accounts to your own tribe to fill in missing monsters that you require for your Tribe.
As for Dawn of Fire, you can make one of your accounts "donate" coins by selling off Crafting Items cheaper than bought. You can also "donate" Crafting Items via the Market, though that is fairly risky, especially if you have lots of publicity. You can also deliberately make an account request Crafting Items for the Skyship and let your other account(s) fill in the orders and get any of your accounts higher in the ranks. You can also split the work force on your Crafting Item crafters, so that one account focuses on one range of specific types of Crafting Items (such as "cheap" Crafting Items or "expensive-to-produce" Crafting Items, primary Crafting Items, intermediate Crafting Items, and end-product Crafting Items) and one account focuses on another range of specific types of Crafting Items.
Disadvantages of Multiple Accounts/Advantages of Single Accounts
To have multiple accounts is difficult to handle to control which account gets Key requests. It is also more difficult to decide which account to earn the rewards from any given contest by Big Blue Bubble. It is also more time consuming to update every single game you possess, so be sure to have plenty of time before having multiple accounts of My Singing Monsters and/or Dawn of Fire.