Avi, thanks for pointing out that Tiqun Simanim uses a somewhat different symbol for pashta and qadma. I'm amazed that I never noticed that before --- but maybe I shouldn't be so amazed considering that I've been conditioned for so many years to believe that these tropes are identical in appearance, so there was no reason for my brain to internalize the difference. Each time I saw either one, I "recognized" the symbol as the one that (in my mind) represented both qadma and pashta, so there was no need to do any further processing.
We all know that when a pashta appears on a mil`eil word, virtually all xumshim have two pashtas --- the first one to indicate where the accent is and the second one (at the end of the word) to indicate that it's a pashta rather than a qadma. Well, once there are two different symbols, there would be (at least in theory) no need for the second pashta. Of course, even Tiqun Simanim doesn't go to this extreme --- it would be too confusing to those ignorant people (like me before today!) who are oblivious to the use of the two different symbols. However, I noticed something weird --- in such words, the first "pashta" is displayed in Tiqun Simanim using the symbol for qadma! I wonder why --- it seems to me that two pashtas would be more appropriate than a qadma and a pashta. But I'll propose a possible answer in what follows.
Thanks to Jay for pointing out v"yasafta (D"varim 19:9) and to MG for pointing out vahareimota (B"midbar 31:28). On top of that, I realized that there are probably many examples of both pashta and qadma on the word l"kha. I certainly didn't look for lots of them --- just thought of one of each off the top of my head, namely asei l"kha teivat atzei gofer (pashta --- B"reishit 6:14) and v"haya l"kha l" 'ot al yad"kha (qadma --- Sh"mot 13:9). It turns out that Tiqun Simanim places two of the qadmas (D"varim 15:18 and 19:9) on the extreme right hand side of the last letter of the word and the other two qadmas (B"midbar 31:28 and Sh"mot 13:9) near the middle of the last letter of the word. At this point, I thought that Tiqun Simanim was inconsistent --- but I was wrong. When I looked up the pashta in B"reishit 6:14, I discovered that this pashta and all others (i.e., even the ones where the last syllable consists of more than one letter) is placed on the far left side of the last letter of the word, so that a small part of it is "posheit" (as someone has already pointed out) beyond the entire word.
At this point, I realized two things.
(a) Simanim is perfectly consistent in distinguishing between qadma and pashta when the last syllable contains just one letter --- pashta is on the extreme left side of the letter, while qadma is sometimes on the extreme right side of the letter and sometimes in the middle of the letter. It's strange that there's not more consistency between different qadmas --- but there's a perfectly ironclad rule for distinguishing pashta from qadma based purely on location (even if you ignore the use of the two different symbols).
(b) Perhaps the desire to restrict the pashta symbol only to the very end of a word (and beyond) is the reason why they chose to use the qadma symbol for the first "pashta" in words that require two of them. Nevertheless, I would personally prefer to see a pashta symbol in the middle of a word that has two of them than to see a qadma symbol on a word that clearly has no qadma associated with it.
Art