Allow one-mon double battles (#4007)

Co-authored-by: Alex <93446519+AlexOn1ine@users.noreply.github.com>

include/config/overworld.h

@@ -5,8 +5,9 @@
 #define OW_RUNNING_INDOORS          GEN_LATEST  // In Gen4+, players are allowed to run indoors.
 
 // Other settings
-#define OW_POISON_DAMAGE            GEN_LATEST // In Gen4, Pokémon no longer faint from Poison in the overworld. In Gen5+, they no longer take damage at all.
-#define OW_TIMES_OF_DAY             GEN_LATEST // Different generations have the times of day change at different times
+#define OW_POISON_DAMAGE                GEN_LATEST // In Gen4, Pokémon no longer faint from Poison in the overworld. In Gen5+, they no longer take damage at all.
+#define OW_TIMES_OF_DAY                 GEN_LATEST // Different generations have the times of day change at different times.
+#define OW_DOUBLE_APPROACH_WITH_ONE_MON FALSE      // If enabled, you can be spotted by two trainers at the same time even if you only have one eligible Pokémon in your party.
 
 // PC settings
 #define OW_PC_PRESS_B               GEN_LATEST // In Gen4, pressing B when holding a Pokémon is equivalent to placing it. In Gen3, it gives the "You're holding a Pokémon!" error.

src/pokemon.c

@@ -3080,6 +3080,9 @@ u8 GetMonsStateToDoubles_2(void)
     s32 aliveCount = 0;
     s32 i;
 
+    if (OW_DOUBLE_APPROACH_WITH_ONE_MON)
+        return PLAYER_HAS_TWO_USABLE_MONS;
+
     for (i = 0; i < PARTY_SIZE; i++)
     {
         u32 species = GetMonData(&gPlayerParty[i], MON_DATA_SPECIES_OR_EGG, NULL);