Hi,

"Stuart Levy" writes:
I browsed a bit but couldn't find any on-line references suggesting

The Journal of the British Astronomical Association has an article on
this in its current issue (Vol 114 No. 3). It is not available online
however http://www.britastro.com/jbaa/114-3.htm

So summarise, they list three simeltaneous and near similtaneous
transits, though none of us will be witnessing them!:

13,425 (the year)
Sept 17th 0345 Venus
Sept 17th 1927 Mercury

69,163
July 26th 1647 Venus
July 26th 2031 Mercury

224,508
Mar 27th 2259 Venus
Mar 28th 0339 Mercury

specific values for rates of node advance for Mercury or Venus.

From the 1992 edition of the Explanatory Supplement to the
Astronomical Ephemeris, p. 316, we find for J2000 for mean equinox and
ecliptic of that date:
Mercury's node = 48.33° - 4.4630"/year
Venus's node = 76.68° - 9.9689"/year

However, the inclinations are not constant, so these are not
representative of the long-term behavior.

They mention this in their paper and state that to account for this they
used formulae from Simon J L et. al, `Numerical expressions for
precession formulae and mean elements for the Moon and the planets`
Astron.Astrophysics 282, 663-683 (1984) [
http://adsabs.harvard.edu/cgi-bin/np...6A...282..663S
]

I hope this is of help.

Cheers,
~Al
--
www.irishastronomy.org