Again On Themed Anthologies

Poul Anderson wrote:

a US Robots story, published in an Asimov-themed anthology (see here);
a Time Patrol story, published in a Knights Templar themed anthology (see here).

Isaac Asimov wrote an installment of his own Black Widowers series, published in a Batman-themed anthology (see here).

SM Stirling edited a Drakas anthology and wrote:

a Lords of Creation story, published in a retro-Mars anthology;
an Angrezi Raj story, published in an alternative histories anthology (see here);
a Time Patrol story, published in an Anderson-themed anthology (see here);
a John Carter story, published in a Barsoom-themed anthology (see here).

Thus, through the medium of themed anthologies, Anderson contributes to Asimov's future history and to his own Time Patrol series whereas Stirling contributes to:

two of his own alternative histories, including his own retro-Mars;
Anderson's Time Patrol series;
ERB's John Carter series, set on ERB's Mars.

Complicated. I am about to read Stirling's John Carter story.