I'm not 100% sure I completely understand your question. Most people want reserve artillery battalions so they can field 152mm howitzers. The short rule of thumb: For every 152mm gun in your list, Strelkovy lists must field a 122mm gun in a separate artillery battalion, and Motostrelkovy lists must field a 76mm ZIS field gun in a separate Motostrelkovy artillery battalion.

Assuming you are fielding a Strelkovy list from Fortress Europe:

You say you have two howitzer platoons in a regular artillery battery already, or four guns. So you can have a maximum of four guns in the reserve battalion, and you have to have at least four guns in a reserve arty battalion, so if you field a reserve arty battalion, it will have exactly four guns.

That would give you two artillery battalions, each at four guns. The four guns from the regular artillery battalion would be 122mm, the four guns from the reserve arty battalion could be 122mm or 152mm.

You specifically CANNOT deploy two reserve artillery battalions. On Strelkovy lists, page 140 only has one box with the words 'Reserve Artillery Battalion' listed. Same for Motostrelkovy on page 134.

Yes, you can do that, but in two separate battalions.  (Shudder - not looking forward to facing off against THAT.)