The dual nature of the contemporary orchestral concert experience was clearly displayed last Friday and Saturday nights by the Boston Symphony concerts at Tanglewood. Each offered its own image of how an audience can interact with familiar works. Each featured a central European conductor leading core repertory: one concerto and one symphony each, all works familiar to habitual concert-goers. Each pair of pianists and conductors exhibited strongly-marked contrasts. Both concerts were satisfying, but in very different ways and to markedly different degrees.