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I've placed seams and have unwrapped the two selected objects in the picture below.
However, whenever I unwrap the objects the UV map is considerably bigger than the texture tile and I am unable to sufficiently scale or pack the objects enough to prevent "oversized" textures.
(Blend File)
The game engine I'm importing into cannot support UDIM texturing.
The UV islands are basically like vectors, the actual scale of them in the viewport doesn't correlate to the resolution. As long as your texture has a high enough resolution, it should be fine. You can use UDIMs if you have too many shells to fit into one square, which basically means that instead of fitting all the shells into one square texture, you tile your uvs so you can have multiple square textures next to each other, then you can make your shells as big as possible. You also TECHNICALLY can use textures that aren't square if it's just gonna stay in blender, but it isn't best practice and I don't think you can use UDIMs then. Also look up texel density, there is a free addon that checks/sets the texel density of your shells so everything will be the same resolution on the texture.
If you need uniform UV size across multiple objects for tileable textures to be the same size, you can unwrap multiple objects at once. You can select multiple objects, go into edit mode, select all and unwrap. Note that each object is unwrapped taking its scale into consideration, so if you want the UVs to be uniform size on the surface in terms of world space, you have to apply scale( Ctrl + A -> Scale in object mode) to the objects.
If you then need different kinds of textures you can also use multiple UV maps for different textures:
You will have to choose which UV map to use for which texture in your shaders. Most software and 3d formats supports multiple UV maps.
If you want to control the scale of tileable textures with UVs only, you can also scale the UVs way bigger than the (0,0)-(1,1) UV space. Most software tiles the UV space by default, but you can usually control that from your shaders as well.
If you need more resolution for your textures, there is no magic way to solve the issue without using higher resolution textures or more textures. UDIM is an easy way to manage assigning multiple textures to the same object, but it is not necessary for you to be able to do that. You can have multiple UV maps and work with your shaders to use multiple textures for the same object, it's just more work and less convenient to set everything up. It's easy to do that with Blender nodes, but Blender's material nodes usually only work inside Blender and will not export to game engines correctly so how you would do that depends on the game engine you are using and how shaders work in it.
