How do I calculate "melded" or "efective" rates on multiple loans? For
example, if I'm paying a 5% rate on a $100,000 mortgage and 9% on a
$50,000 mortgage and both are at terms of 360 months, how do I
determine what rate I'm paying on the entire $150,000? And then how
would I do so if the terms were different: say 240 & 360? Thanks

You have 2 loans:

Loan 1: PV = $100,000
        Interest/Month = 5/12
        Length of Loan = 360 Months
        Monthly Payments = $536.82
Loan 2: PV = $50,000
        Interest/Month = 9/12
        Length of Loan = 360 Months
        Monthly Payments = $402.31

Total Monthly Payments = $939.13
Total Amount Paid over the loan = $939.13 * 360 
                                = $338,086.80


Total Interest = 188,086.8
Interest on Loan 1 = 93,255
Interest on Loan 2 = 94,831.6

5 * 93,255/188086 + 9 * 94,831.6/188,086
= 7.01%

For Different Time periods, in a similar manner, you interest rate will be

Loan 1: PV = $100,000
        Interest/Month = 5/12
        Length of Loan = 240 Months
        Monthly Payments = $659.96

Loan 2: PV = $50,000
        Interest/Month = 9/12
        Length of Loan = 360 Months
        Monthly Payments = $402.31

SIMILARLY CALCULAGE IT FOR THE 2ND CASE ...

This answers my question exactly, thanks so much.

Actually the second one will be a bit more work !

Loan 1: PV = $100,000
        Interest/Month = 5/12
        Length of Loan = 240 Months
        Monthly Payments = $659.96

Loan 2: PV = $50,000
        Interest/Month = 9/12
        Length of Loan = 360 Months
        Monthly Payments = $402.31

Effective Interest over 240 Months for 9% loan
Total Interest = 78314.41

Effective Interest over 240 Months for 5% loan
Total Interest = 58,389

Effective Interest over 120 Months for 9% loan
Total Interest = 16,517

= 7.3% + 0.09 * (16517/153220)
= 7.3 + 0.1 = 7.4%

Thus you calculate the blended for the first 2 and then blend the
third one into it!!