This article extends existing alliance portfolio studies by exploring differences of antecedents for forming simultaneous alliances and one-by-one alliances using data from 1,029 alliances in the global pharmaceutical industry. The research’s findings suggest that firms are inclined to form simultaneous alliances when they have more prior alliance experience, conduct exploration alliances, or are in high level of technological diversification. Second, although simultaneous alliance formation may decrease a firm’s performance, greater alliance experience and inter-industry alliances may mitigate such the negative effect of simultaneous alliance formation on firm performance. Low levels of technological diversification may increase firm performance as forming simultaneous alliances. Moreover, propensity to form simultaneous alliances is increased with the increased number of formed alliances as larger firm size, organizational experience, and alliance experience take place. This article contributes to the current strategic alliances and organizational learning literature by introducing the concept of simultaneous and one-by-one alliances.