Single bridge is the most recommended approach.
Create a VLAN interface attached to bridge, one for LAN, one for Guest.
Then add DHCP server to each of those VLAN interfaces.
You may want to check this excellent material from pcunite, router function.
http://forum.mikrotik.com/t/using-routeros-to-vlan-your-network/126489/1
If you’re still stuck, post config between code quotes and surely someone will have a look.