Have ridden tandems since 2000. We are on our second tandem - the first one was donated to a good cause not long after we got the second/current one.
I've had at least two single bikes for several decades but never more than three. Currently have two recumbent single bikes. Plus the tandem. The singles are quite different - one was my commuter for a decade. Not actively lusting after +1 but would consider a titanium version of my steel recumbent if I stumbled across one. Or just a ti frameset. My other 'bent already had the original steel frame replaced by a ti frame a few years ago. Not completely immune to the call of n + 1.
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Bacchetta Giro A20, RANS V-Rex, RANS Screamer