Bilaterally is the adverb: two groups or two nations doing something together, or a thing being done between those two sides. They agreed bilaterally. The question was talked about bilaterally, not at a big meeting of many countries. Same formal, news-and-government feel as bilateral. You are saying how the action happened — between two parties — not describing the agreement itself. Though actually, you will hear the adjective much more often; the -ly form shows up when a verb needs an adverb, and some writers just keep saying “in bilateral talks” instead.

以雙邊方式、雙邊地