Try this:
Select tblComputers.Computer As Displayname, tblComputers.LastknownIP As
ipaddress, tblCompCustom.Location, tblCompCustom.Department,
tblCompCustom.Building
From tblComputers Inner Join
tblCompCustom On tblComputers.Computername = tblCompCustom.Computername
Union
Select tblcustdevices.Displayname, tblcustdevices.Ipaddress,
tblcustdevices.Location, tblcustdevices.Department, tblcustdevices.Building
From tblcustdevices