I'll assume you are playing SH3, which you can begin with a 1939 type IIa...
Unless you wish to play with the realism of waiting a while for your assigned crew to reload the tubes, you can select "Auto Loading" to instantly load those two spares: click on your 'Weapons Officer' icon in the bottom left panel, then check Auto Loading at top left in his weapons management panel. Hope this is all you need.
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