DC-DC cascade or parallel?
Since Buck/Boost is always more complex and costly, you want to avoid that in any configuration. 
I'd suggest the most cost effective and lowest component count would be:


Boost convert the Li-Ion 3-4.2V supply to 5V. Likely 90%+ efficiency at &lt; 7W
Buck convert the 5V supply to 3.3V. Likely 75%+ efficiency at ~ 1.7W


You may need relatively high output capacitance on the 5V supply as the 3.3V Buck converter will draw pulses of current which depending on the converter IC used could be up to 4A. You will see larger pulse current for lower frequency configurations.

For example, the 3.3V supply could be based on a TPS62237 for ultra low cost and low component count.
Here the switcher is at 2.7MHz and so the pulse input current is reduced to about 0.7A which significantly reduces the required input capacitance for the converter.
This would result in a peak output current on the 5V supply of about 1.7A+. 

Use the TI WeBench Power Designer for details:



I'll leave you to research the options for the Boost converter, but WeBench suggests the lowest cost/component count would be the TPS61022.