There are laws adorning Weapon Fusion, one being that a pair of weapon types will always create a certain weapon-type (ie. Blade x Club = Staff). Additionally, fusing two certain weapons will always result in a certain weapon (ie. Burst Blade x Magnus Club = Orb Staff). This allows for the option of chain-fusing in order to design much more devastating attribute combinations. Below is a chart of weapon type fusions.
There are 12 weapons within each weapon-type, and each of those weapons has their own ID number ranging from 1 to 12. By using the chart above and these numbers, you can calculate the outcome of (almost) every fusion combination. To do this, you simply add the two ID numbers of the weapons you are going to fuse together, and then find the weapon with the corresponding ID number of that sum under the correct resulting weapon-type. If the sum is greater than 12, then subtract 12 to get the correct number (ie. 3 + 10 = 13 - 12 = 1). Each weapon and their ID number is listed below.
As an example, let's assume you want to create a Virgo Palm (9). The weapon-type pairs you would use are Blade x Bow, Blade x Orbitars, Cannon x Cannon, and Orbitars x Arms, and the ID numbers for those weapon you would use are 1 & 8, 2 & 7, 3 & 6, 4 & 5, 9 & 12, 10 & 11, and viceversa.
As such, all weapons categorized under a weapon-type can be used to create any weapon in the resulting weapon-type. If you try to fuse two weapons to make a new one and all you see is an X and a question mark, it means you haven't unlocked that weapon yet. (Refer to this guide to learn how to unlock it.)