Hi Andrew,
There is no hard and fast rule for this as it all depends how closely related the keywords are, the competition etc.
The 'general' advice given is to target every page toward 1 unique keyword AND unique content. This is because ranking factors are such things as The title text, the heading text and it is very hard to put the keywords in these locations without it being spammy and forced. But these keywords should be different and not too closely related.
You should interlink from every page where appropriate to really tell the SERPS that "This is my page on Smarty Programmer" and "This is my page on Smarty Tuner". Do not be tempted to have the same page with tuner replacing programmer. Actually write different equally good content or you bring yourself a whole new set of problems.
Think of it as a model, if you have 10 keywords you want to focus, have 10 landing pages with each optimised to 1 keyword. Then have additional supporting pages in the form of blogs or information articles that are related and link to your landing pages. Your page and supporting pages will naturally attract any longtail keywords available that are less competitive.
There are ofcourse exceptions to this rule. A page can rank for several highly competative keywords, its just not that common.
That said your page ranks really well for both terms already. 1-4th for both. If Smarty Programmer and Smarty Tuner are not very competitive, you may be able to target them with 1 page by doing something such as
<title>Smarty Programmer - The Number 1 tuner for diesel trucks</title>
Smarty Programmer
Content targeted at Smarty Programmer
Why Pick the Smarty Tuner?
Content targeted at Smarty Tuner
Ensure both keywords have at least 1 image with alt and title text targeting them. This example is a bit to cut and dry but demonstrates the idea.
This should allow you to rank for both phrases on the same page. BUT there needs to be enough content supporting each phrase, and a suitable amount of competition. If the competition if too fierce you can just shoot yourself in the foot and not rank for either phrase.
If I was you I would run a couple of tests. Get Smarty Tuner inserted a couple of times within the text. Google is clever and it may associate the words enough to begin ranking you for both without sacrificing your current ranking for smarty programmer. Slowly introduce more content targeting smarty tuner such as an image with its alt text and title text, the a few smaller headings etc and some more text and I recon you can quite easily have the same page targeting both and holding no1.