It seems that boon description there is incorrect. The boon was not limited to one opponent only or to a single man combat. As per the boon given by Brahma to Vali, when confronting an opponent Vali will conquer half of the opponents strength:
आहूय वालिनं ब्रह्मा ददौ वरं अनुत्तमम् । प्रतीपवर्तिनोभूयात् अर्ध बलं अरिन्दम ॥
aahuuya vaalinam brahmaa dadau varam anuttamam | pratiipavarti nobhuuyaat artha balam arindama ||
Summoning Vali, Brahma gave an unexcelling boon... any combatant confronting Vali will loose half of his strength to Vali...
So even if Vali faced two opponents, he would take away half of both of their strengths.
That's why while Vali was busy battling Sugreev, Shri Ram shot an arrow at his chest hiding at a distance without facing him directly. Because if Rama faced him directly then Vali would have taken half of His strength as well and Rama could have been defeated by Vali:
दृश्यमानः तु युध्येथा मया युधि नृपात्मज | अद्य वैवस्वतम् देवम् पश्येः त्वम् निहतो मया ||
dR^ishyamaanaH tu yudhyethaa mayaa yudhi nR^ipaatmaja |
adya vaivasvatam devam pashyeH tvam nihato mayaa || [VR - 4.17.47]
Had you been in combat with me en face oh, prince, you would have been killed by me and by now you would have seen the death-god Yama. [4-17-47]
See the side note here.
