It could well be that belisarius suggestions will lead you to what you'll be using in the end. I think you should still know about the most straightforward way to create a combination of interpolating functions using Piecewise and a pure function:

ipf1 = Interpolation[Table[{x, Sin[x]}, {x, 0, 1, 0.1}]]
ipf2 = Interpolation[Table[{x, Sin[x]}, {x, 1, Pi, 0.1}]]
ipfCombined = Function[Piecewise[{{ipf1[#], # <= 1}, {ipf2[#], # > 1}}]]

the result can almost everywhere be used just like an InterpolatingFunction:

Plot[ipfCombined[x], {x, 0, Pi}]
Integrate[ipfCombined[x], {x, 0, Pi}]

(if you want to show a continuous plot you can add the option Exclusions -> None)

It could well be that one of the other suggestions will lead you to what you'll be using in the end. I think you should still know about the most straightforward way to create a combination of interpolating functions using Piecewise and a pure function:

ipf1 = Interpolation[Table[{x, Sin[x]}, {x, 0, 1, 0.1}]]
ipf2 = Interpolation[Table[{x, Sin[x]}, {x, 1, Pi, 0.1}]]
ipfCombined = Function[Piecewise[{{ipf1[#], # <= 1}, {ipf2[#], # > 1}}]]

the result can almost everywhere be used just like an InterpolatingFunction:

Plot[ipfCombined[x], {x, 0, Pi}]
Integrate[ipfCombined[x], {x, 0, Pi}]

(if you want to show a continuous plot you can add the option Exclusions -> None)

It could well be that belisarius approach is what works best for you. I think you should still know about the most straightforward way to create a combination of interpolating functions using Piecewise and a pure function:

ipf1 = Interpolation[Table[{x, Sin[x]}, {x, 0, 1, 0.1}]]
ipf2 = Interpolation[Table[{x, Sin[x]}, {x, 1, Pi, 0.1}]]
ipfCombined = Function[Piecewise[{{ipf1[#], # <= 1}, {ipf2[#], # > 1}}]]

the result can almost everywhere be used just like an InterpolatingFunction:

Plot[ipfCombined[x], {x, 0, Pi}]
Integrate[ipfCombined[x], {x, 0, Pi}]

(if you want to show a continuous plot you can add the option Exclusions -> None)

It could well be that belisarius suggestions will lead you to what you'll be using in the end. I think you should still know about the most straightforward way to create a combination of interpolating functions using Piecewise and a pure function:

ipf1 = Interpolation[Table[{x, Sin[x]}, {x, 0, 1, 0.1}]]
ipf2 = Interpolation[Table[{x, Sin[x]}, {x, 1, Pi, 0.1}]]
ipfCombined = Function[Piecewise[{{ipf1[#], # <= 1}, {ipf2[#], # > 1}}]]

the result can almost everywhere be used just like an InterpolatingFunction:

Plot[ipfCombined[x], {x, 0, Pi}]
Integrate[ipfCombined[x], {x, 0, Pi}]

(if you want to show a continuous plot you can add the option Exclusions -> None)