Mercado P&x00FA;blico is a ChileCompra's platform for tenders, an environment where Chilean companies offer and demand goods and services of all kinds. The main aim of this platform is to make more efficient and transparent the contracting processes. Nowadays, tenders asking more than one good and/or service are answered separately. Thus, after receiving the offers from the sellers, the buyer must select for each part of the tender the best offer. Unfortunately, this is an inefficient and non optimal process. On the one hand, buyers spend much time in building a joint contract where not necessarily they obtained the best deal and on the other hand, small providers are less likely to be selected (if they apply) when competing with bigger providers or alliances of them (which already offer most products and typically at better prices). To address these issues, we presented VirtualMarket, a multi-agent platform, which extends the ChileCompra's platform representing providers as software agents (with willingness to cooperate), which, through the creation of alliances, are capable of naturally identifying associativity opportunities, when tenders are published, and negotiate pre-agreements which make them more competitive. In this ongoing work, we propose to use a slightly modified version of the Zeuthen Strategy in order to support providers agents during the negotiation of the terms of the alliance formation without revealing their utility functions. Our preliminary results about the feasibility and performance of our proposal are encouraging.