Tom Swift crossovers
The Hardy Boys and Tom Swift crossed paths in only one series: the short-lived Hardy Boys and Tom Swift Ultra Thrillers, by Franklin W. Dixon. The two volumes, Time Bomb and The Alien Factor, were published in 1992 and 1993. Despite the creators' use of the Franklin W. Dixon pseudonym, the Ultra Thrillers were definitely more sci-fi than mystery, tying in with the Tom Swift paperbacks that Archway was publishing at the same time.
1. Time Bomb (1992)
As you might expect, Time Bomb is a tale of time travel. The book makes use of a tried-and-true sci-fi time-travel cliche: the final chapter begins as an exact repetition of the first chapter.
2. The Alien Factor (1993)
The Alien Factor involves "a visitor from outer space who is as beautiful as she is strange" (quoting the back cover). The boys' time-travel exploits in the previous book have conveniently wiped away their memories of the time they spent together, making them near-strangers once again. This allows Frank and Joe to be initially suspicious and distrustful of the Swifts.