Sodium seleno-sulphate is analogous to sodium thiosulphate with selenium replacing one sulphur atom. This is something I copied from the web years ago, posted by someone called Liam. I'm sorry I can't make a more complete attribution. The thiosulphate apparently accelerates the toning, and makes the toner more stable.
Originally Posted by Gerald C Koch
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a bit of info from Clerc:
The use of selenium in the form of seleno-sulphate* (with ammonium chloride
added) for toning print-out papers was suggested in 1912 by P. Rehlander as
being more economical than toning with gold and platinum. [Clerc says
elsewhere that the same solution is fine for development papers.] Even with
papers that have been well washed to remove the soluble silver salts, toning
with sodium selenosulphate tends to stain the whites of the image very
strongly. This trouble can be obviated, when toning is done after fixing
and washing, by adding a small quantity of sodium hyposulphite [what they
then called sod. thiosulphate, or hypo] to the toning solution (Lumiere and
Seyewetz, 1924). This procedure greatly increases the speed of toning. The
tones obtained vary from dark brown to red**, according as the image is
coarse or fine grained; this toning gives (with certain papers) an
appearance of platinum toning when used on a print which has been toned with
gold and fixed.
Sodium selenosulphate may be obtained by dissolving 260 gr. of powdered
selenium in 20 oz. (30 grm. in 1,000 c.c.) of a warm 20 per cent solution of
anhydrous sodium sulphite.
Twenty to 50 minims of this solution are added to 20 oz. of a 30 per cent
solution of hypo (2 to 5 c.c. per litre of hypo solution). The smaller
quantity is used for toning albumen papers and the larger for emulsion
papers (gelatine or collodion P.O.P.). The bath is limpid, colourless, and
Toning is very rapid, taking from 2 to 5 minutes, according to the degree of
exhaustion of the bath. This toning solution is very economical; 35 oz.
(1,000 c.c.) will tone 80 prints 7 X 5 in. in size, and the results are
highly permanent if washing is properly done.
* Selenium, which in many of its properties resembles sulphur, appears
usually as grey crystalline masses; in a fine state of subdivision it is
generally red. The alkaline selenosulphates, and particularly the sodium
salt (Na2SSeO3), have a constitution which is closely analogous to that of
the hyposulphites (Na2S2O3), the selenium replacing a portion of the
sulphur. Just as thiosulphate is obtained by dissolving sulphur in a hot
solution of sulphite, so selenosulphate is obtained by dissolving selenium
in hot sulphite solution. As selenosulphate is not made commercially it
must be prepared as required by dissolving selenium. Solutions of
selenosulphates should be kept in full bottles, well stoppered to avoid
conversion into inactive selenotrithionates by oxidation; they are more
stable in presence of sodium sulphite or hyposulphites. Analogous compounds
of tellurium have also been used for the toning of silver prints.