Usually, a single 2x teleconverter is the better choice than stacking two 1.4x converters—assuming the converters are of similar quality.

There’s no absolute rule because it depends on the specific optical designs. Two excellent 1.4x converters could theoretically outperform a poor 2x converter, but in general a single 2x will beat two stacked 1.4x units.

Why? It’s both magnification and added optics:

• More magnification enlarges not only the subject, but also the lens’s existing optical flaws.
• Adding more glass usually introduces more degradation unless that extra glass provides some corrective benefit.

So with two 1.4x converters, you’re typically getting the downside of extra optical elements without an advantage over a properly designed single 2x converter.

In short: if you need about 2x reach, a good dedicated 2x teleconverter is normally the preferable option over stacking two 1.4x teleconverters.