A proposal from FCC Chairman Tom Wheeler would block an attempt by Sprint and T-Mobile US to buy spectrum together in the incentive auction that will transfer airwaves from broadcast TV stations to cellular carriers next year. Announced on Friday, Wheeler’s proposal seeks to help the smallest wireless companies develop business partnerships with larger ones. But it would not allow partnerships between the biggest carriers, since more than 95 percent of US customers are served by either AT&T, Sprint, T-Mobile, or Verizon Wireless. “Our goal is to promote the participation of as many parties as possible in the auction,” FCC Wireless Telecommunications Bureau Chief Roger Sherman wrote Friday. “If two of the largest companies are able to bid as one combined entity in the auction, their combined resources may have the effect of suppressing meaningful competition.